1. Field of the Invention
The present invention relates to a conductive member which is disposed in the vicinity of the image carrier of an image forming apparatus of an electrophotographic type and is used as a charging member, transfer member, or the like, and in particular, it relates to a charging roller, process cartridge and image forming apparatus which use such a conductive member.
2. Description of the Background Art
Conventionally, in an image forming apparatus of an electrophotographic type, such as a copying machine, a laser beam printer, or a facsimile machine, conductive members are used as a charging roller which applies charge to an image carrier (also called “photosensitive body” below), and as a transfer. roller which performs transfer processing of toner on the photosensitive body.
For example, as prior art technology relating to an image forming apparatus using a charging roller of this kind, Japanese Patent Application Publication No. S63-149668 and Japanese Patent Application Publication No. H01-267667 disclose a contact charging method in which a charging roller is placed in contact with the photosensitive body. However, a contact charging method involves problems of the following kind.
Namely, the material constituting the charging roller seeps out from the charging roller and becomes attached to the surface of the charged body, giving rise to a charging roller trace which adheres to the surface of the charged body. In particular, the seeping described above makes toner adherence more liable to occur. Furthermore, the charging roller which makes contact with the charged body oscillates when an AC voltage is applied to the charging roller, and this creates a charging sound. Moreover, the toner on the photosensitive body becomes attached to the charging roller, and this causes charging performance to decline. Yet further, if the photosensitive body is halted for a long period of time, the material constituting the charging roller becomes attached to the photosensitive body and the charging roller suffers permanent deformation.
As technology for resolving problems of this kind, a proximity charging method, in which the charging roller is placed in the proximity of the photosensitive body, has been proposed instead of the contact charging method described above. More specifically, Japanese Patent Application Publication No. H03-240076 discloses a proximity charging type of charging roller in which the proximity gap between a charging roller and a charged body is set to 5 to 300 μm, an outer layer made of EPDM, or the like, whose resistance has been reduced to a prescribed extent by using carbon, or the like, is provided on the outer side of a conductive metal core, and spacer rings made of nylon, tetrafluoroethylene (product name: Teflon (registered trademark)), or the like, are provided in an integrated fashion on either side end section of the roller and extending in the circumferential direction of the roller. Furthermore, Japanese Patent Application Publication No. H04-358175 discloses a proximity charging type of charging roller in which the proximity gap between a charging roller and a charged body is set to 1 mm or less, the roller comprises a conductive metal core and a resistance layer, and spacer rings made of an insulating material are provided in an integrated fashion on either side end section of the resistance layer.
In proximity charging methods of this kind, the charging roller and the photosensitive body are mutually opposing in such a manner that the closest distance (gap) between same is 50 to 200 m, and by applying a voltage to the charging roller, the photosensitive body becomes charged. In a proximity charging method, since the charging apparatus and the photosensitive body do not make contact with each other, then there is no occurrence of problems which arise in a contact charging method, such as adherence of the material constituting the charging roller to the photosensitive body, or permanent deformation occurring due to a prolonged idle period of the photosensitive body. Furthermore, the proximity charging method is also advantageous in respect of the problem of degraded charging performance due to adherence of toner, or the like, from the photosensitive body to the charging roller, since the amount toner adhering to the charging roller is reduced.
However, although the proximity charging method has is the advantages described above in comparison with a contact charging method, it involves the following two problems, which make practical application difficult. More specifically, in the proximity charging method, it is difficult to maintain a uniform gap between the charging member and the photosensitive body, and furthermore, charging non-uniformities are liable to occur due to variation in the gap between the charging member and the photosensitive body. When charging non-uniformities occur, they can give rise to image defects, such as adherence of toner to white areas of the paper. With respect to the problem of maintaining a uniform gap in the section of closest proximity between the charging member and the photosensitive body, in order to prevent the occurrence of image defects due to charging non-uniformities, the variation in the size of the gap in the section of closest proximity between the charging member and the photosensitive body must be restricted to approximately 20 μm, for example.
As a device for maintaining the gap between the charging roller and the photosensitive body, Japanese Patent Application Publication No. 2001-296723, for example, discloses a non-contact charging apparatus in which a jointed tape-shaped member is installed as a gap management member on the outer circumference of either end section of an elastic roller section, thereby forming a gap with respect to the surface of an image carrier, and although it resolves the problems described above in the short term, the elastic rubber used for the charging roller is liable to suffer flatting over time, and with use over a long period, it is not possible to maintain the gap between the photosensitive body and the charging roller. Furthermore, with a long period of use, factors such as abrasion of the tape-shaped member, infiltration of toner in between the charging roller and the tape-shaped member, and adherence of solid toner to same, and the like, make it impossible to maintain the gap between the photosensitive body and the charging roller.
Furthermore, Japanese Patent Application Publication No. 2004-354477 discloses a configuration in which a thermoplastic resin composition having a durometer hardness of HDD 30 or above or HDD 70 or below, and wear characteristics in a Taber abrasion tester of 10 mg/1000 cycles or less, is used as a gap maintaining member, and gap maintaining members of this kind are pressure-fitted onto either end section of a roller. According to this composition, as described in more detail below with respect to the drawings, long-term reliability is improved by means of the tape-shaped gap maintaining members.
Japanese Patent Application Publication No. 2005-076138 discloses technology for simultaneous processing, in other words, simultaneous removal processing of a gap maintaining member and an electrical resistance adjusting layer, whereby it becomes possible accurately to control the gap between a charging roller and a contacting member against which it is abutted, for example. However, if the electrical resistance adjusting layer and the gap maintaining member are made of different materials, then due to the difference in their water absorptivity, there will be a difference in the amount of dimensional variation they experience with change in the ambient conditions, thus leading to the problem of variation in the size of the gap. More specifically, the gap maintaining member and the electrical resistance adjusting layer are usually formed from different materials, in consideration of the adherence characteristics of solid toner, but since an ion-conductive agent is used as the resistance adjusting agent of the electrical resistance adjusting layer, then it has high water absorptivity, and in high-temperature and high-humidity conditions, the electrical resistance adjusting layer absorbs moisture and is liable to experience dimensional change. On the other hand, the gap maintaining member is desirably made of an olefin-based material, in view of its insulating properties and resistance to adherence of solid toner, but the olefin-based material has low water absorptivity and undergoes little dimensional change in high-temperature and high-humidity conditions, compared to an electrical resistance adjusting layer. Consequently, there is a problem in that the gap (step difference), which has been formed with high precision, will suffer variations as a result of changes in the ambient conditions.
In order to resolve problems of this kind, Japanese Patent Application Publication No. 2005-019517 proposes a composition in which a step section is provided in one or more steps in the vicinity of either end of the electrical resistance adjusting layer, as described hereinafter with reference to the drawing, and a gap maintaining member is abutted against and fixed to two or more of the faces constituting each step section of the electrical resistance adjusting layer. However, when carrying out removal processing, such as cutting and polishing, on the gap maintaining member, and especially if the gap maintaining member has a small thickness, peeling or pulling of the end sections of the gap maintaining member occurs due to the stress created by the cutting blade and there is a risk that the shape of the gap maintaining member will be deformed, thus causing variations in the size of the gap.
Technologies relating to the present invention are also disclosed in, for example, Japanese Patent Application Publication No. H03-052058 and Japanese Patent Application Publication No. H06-093150.